The phenomenon of acoustic droplet vaporization (ADV), the acoustically mediated phase transition of liquid perfluorocarbon droplet emulsions into gas bubbles, is under investigation for several biomedical applications. For example, submicron-sized perfluorocarbon droplets extravasate from leaky tumor vessels, undergo ADV, and provide contrast on ultrasound images of cancerous tissue. ADV-induced microbubbles generated from micron-sized droplets have also been investigated as point targets for phase aberration correction. Further, contrast-enhanced photoacoustic images have been created using ADV to trigger the localized release of cardiogreen dye from a perfluoropentane (PFP) double emulsion. ADV is also being explored for various direct and adjuvant therapeutic applications. Micron-sized perfluorocarbon microbubbles created via ADV have been shown to occlude capillary beds and arterioles, which can facilitate embolotherapy in cancer treatment. Previous studies have also demonstrated ADV-mediated delivery of chemotherapeutic drugs such as paclitaxel, chlorambucil, and doxorubicin loaded in perfluorocarbon droplets. Thermal ablation of cancerous lesions has been enhanced using perfluorocarbon droplets as cavitation nuclei during high intensity focused ultrasound (HIFU) exposure. Further, ADV microbubbles can provide contrast-enhanced image guidance during treatment. More recently, perfluorocarbon droplets have been shown to lower the acoustic power needed for HIFU-mediated lysis of blood clots relative to HIFU exposure without droplets. Spatiotemporally-controlled ADV-mediated drug release from perfluorocarbon double emulsions has also been shown to regulate the mechanical properties of tissue-engineered scaffolds.
Several studies have reported that ADV results in a significant volumetric expansion of the perfluorocarbon. For example, the ideal gas law was previously used to predict a volumetric expansion of a factor of 125 (i.e., a radial expansion of 5). The measured volumetric expansion of evaporating perfluoropentane was a factor of 151 (i.e., a radial expansion factor of 5.3), and the computed radial expansion was a factor of 5.9, which accounts for the diffusion of gases into and out of the microbubbles. From these results and other studies, the amount of ingassing and thus the expansion factor appear to depend on the experimental conditions. The diffusion of gases into perfluorocarbon-based ultrasound contrast agents has also been discussed in other studies, while the high solubility of oxygen in perfluorochemicals and the use of perfluorochemicals as blood substitute agents have been demonstrated in previous studies.
Although studies have demonstrated that dissolved gases in a fluid can diffuse in and out of perfluorocarbon droplets, the need exists to develop therapeutic methods of utilizing the phenomenon of ultrasound-mediated gas diffusion concomitant with the cavitation nucleation associated with ADV as a strategy to scavenge gases in situ.